Exothermic welding is widely used in making electrical connections or reinforcing bar splices, for example. A wide variety of connections such as copper-to-copper or copper-to-steel, or aluminum-to-aluminum, as well as the apparatus for making such connections, are sold by ERICO, Inc. of Solon, Ohio under the registered trademark CADWELD.RTM.. The connections made are very low impedance connections and the ampacity is usually higher than that of the conductor or cable.
The apparatus involves a crucible and molds, the latter forming a cavity receiving the parts to be welded. The molds and crucibles may be formed in the same block of ceramic or graphite material, or in separate blocks such as seen in the copending application of Richard Singer et al., Ser. No. 08/381,640 filed Jan. 31, 1995, and entitled "Exothermic Welding Jig, Molds Therefor and Method".
Whether separate or single blocks are employed, the crucible includes a somewhat inverted frusto-conical cavity to contain the exothermic material. At the bottom of the cavity is a seat for a fusible steel disk which supports the exothermic material and below the disk is a tap hole which leads to the mold. The larger upper end of the crucible is closed by a cover. A starting powder is placed over the top of the exothermic material and may be ignited either with a flint gun or electrically. In another version of the process, the crucible is covered by a filter or baffle in a low emission process as sold by ERICO, Inc. under the registered trademark EXOLON.RTM..
Exolon does not require starting material for most applications. An electric start system is used.
The crucible and molds are cleaned after use and reused. If properly cared for and cleaned, they can be used over and over again. With a separable crucible such as shown in the noted copending application, the same crucible may be used with many different size and types of connections. With different size and types of connections, the amount of exothermic material employed may vary widely. More material than necessary should not be employed for a variety of reasons, one of which is waste.
In the process, the molten metal separates from the slag and any slag or excess metal forms in a riser above the connection which is normally removed. In an ideal reaction, the mixture is ignited uniformly over the top area or diameter of the weld metal mass and proceeds downwardly to fuse the disk after a very brief delay. This allows time for complete reaction and for the slag to form and separate to the top of the weld metal. Otherwise, unreacted material or slag may be drawn into the mold adversely affecting the quality of the weld. This ideal condition is adversely affected if ignition does not occur uniformly over the top surface, but rather at one side or the other of the mixture of weld metal. The reaction then proceeds both horizontally across the diameter of the charge and vertically toward the disk. It can then be seen that if the diameter is significantly larger than the vertical height above the disk, fusing of the disk before a complete reaction of the mixture is more apt to occur. Thus, for assured quality welds, the ratio of the diameter of the weld metal volume to the height of the weld metal volume above the disk should be within a certain range.
Crucibles with a single chamber configuration are not suitable for welding connections of different sizes, since the chamber configuration is selected with a given discrete volume of exothermic material in mind to achieve the desired diameter to height ratio. If the volume varies, the ratio changes. For example, a crucible chamber designed for one size weld metal volume may not be suitable for another smaller amount since the height may be considerably less even though the diameter is only slightly smaller. The ratio, however, has changed significantly. This makes the crucible unsuitable for use with smaller molds requiring a larger inventory of crucibles and molds than would otherwise be required.
It would, accordingly, be desirable if a single crucible could accommodate various volumes (masses) of exothermic material while still maintaining the proper configuration of the volume of material to keep the diameter to height ratio within the proper range assuredly to form quality welds.